Ind. Eng. Chem. Res. 2000, 39, 3121
3121
Response to Professor Yablonsky Octave Levenspiel Chemical Engineering Department, Oregon State University, Gleeson Hall 103, Corvallis, Oregon 97331-2702
Sir: Prof. Yablonsky raises two objections to my presentation. First, let me explain my reluctance to use the Langmuir-Hinshelwood-Hougen-Watson (LHHW) equations because “they are too complicated”. If, for example, a researcher comes up with a LHHW equation to represent his porous catalyst data, then I probably could come up with five different other LHHW equations which could fit the data equally well.1,2 Because these equations extrapolate differently, which should I use for extrapolation? I do not know. Even if I cannot find any other LHHW equation to fit his data, let me ask whether he can use his equation with confidence to extrapolate for scale-up? Does his equation account for film diffusion or how about diffusional resistance within the particles? What about film heat transfer at the surface of the particles? Finally, how about nonisothermal gradients within the particles? The LHHW equations completely ignore these physical factors; hence, they are not useful for extrapolation. They simply fit the data. On the other hand, the chemical reaction engineering (CRE) approach accounts for all of the pertinent factors, and that is why in the marketplace of ideas it won out over the “petrotech” approach, which uses the LHHW kinetics. The second objection concerns my ignoring extremely fast reactions. Researchers of such systems use basketfuls of elementary reactions, hundreds and hundreds of them,3 to study free-radical reactions, flames, burners, and other such devices, with computational fluid dynamics (CFD) to represent the flow and mixing. This
type of problem is dealt with primarily by combustion specialists. I see their goals as differing from CRE, whereas Prof. Yablonsky does not. I suppose that this is a matter of opinion. Prof. Yablonsky also says that I do not account for the correspondence between model and goal. I think that the goal of CRE is to come up with a good reactor design to produce efficiently what you want to produce. Kinetic models which use basketfuls of elementary reactions or LHHW mechanisms, but then ignore pertinent factors and at the same time are unnecessarily complicated, are really unsuited to the above-mentioned goal. In conclusion, Prof. Yablonsky says that “we have to live with unavoidable complexity of reactions” “...complexity of equations”. I am uncomfortable with such statements. I feel that we should try to come up with simple models which do account for all of the pertinent factors. As Denbigh4 so wisely put it, “It is always necessary to abstract from the complexity of the real world...and in its place...to substitute a more or less idealized situation that is more amenable to analysis.” Literature Cited (1) Chou, C. H. Ind. Eng. Chem. 1958, 50, 789. (2) Hougen, O. A.; Watson, K. M. Chemical Process Principles; Wiley: New York, 1947; part III, pp 943-958. (3) Rojnuckarin, A.; et al. Ind. Eng. Chem. 1996, 35, 683-696. (4) Denbigh, K. G. The Thermodynamics of the Steady State; Methuen’s Monographs on Chemical Subjects; London, 1951.
IE001095G
10.1021/ie001095g CCC: $19.00 © 2000 American Chemical Society Published on Web 06/24/2000
